1. Field of the Invention
The present invention relates to a toner for developing an electrostatic image used for image forming of an electrophotographic system typified by a copying machine and a printer.
2. Description of the Related Art
Addition of an external additive having a large particle diameter has been proposed as a technique to improve the transferability of a toner (refer to Japanese Patent Laid-Open No. 2007-171666). This proposal is directed to improve the transferability by adding silica particles having large particle diameters to toner particles and reducing physical adhesion between the toner and a photo conductor. However, the silica particles, which are used in this proposal and which have large particle diameters, are particles obtained by a deflagration method and, therefore, have a wide particle size distribution. Consequently, in the case where the toner is used over a long term, particles having large particle diameters are eliminated from the toner easily, and particles having small particle diameters are embedded in toner particles easily. Furthermore, when silica particles move to concave portions of toner particle surfaces, it is difficult to give stable chargeability, fluidity, and transferability to the toner.
As for a technique to improve this harmful effect, addition of silica particles having large particle diameters with a narrow particle size distribution to toner particles has been proposed (refer to Japanese Patent Laid-Open No. 2007-322919). Japanese Patent Laid-Open No. 2007-322919 discloses that silica particles which have large particle diameters with a sharp particle size distribution and which are produced by a sol-gel method are used, so as to improve long-term stable chargeability and improve transferability. However, silica particles, which have large particle diameters and which are obtained by the sol-gel method, in the related art are hydrophilic particles having silanol groups to a large extent. Therefore, even when a hydrophobizing treatment is performed, there are large amounts of remaining silanol groups. Consequently, the property of the silica particles to give the chargeability to the toner is influenced by the temperature and the humidity easily, and it is difficult to give stable chargeability to the toner. Meanwhile, in the case where large amounts of hydrophobizing agent is used for silica particles in order to improve this harmful effect, a property to give fluidity to the toner is degraded. Consequently, in the case where a toner including such silica particles is used and images are output over a long term, it is difficult to maintain high image quality.
As for a technique to improve these harmful effects, use of silica particles, which have silanol groups to a relatively small extent and which have large particle diameters with a specific particle size distribution, to a toner has been proposed (refer to Japanese Patent Laid-Open No. 2008-262171). However, silica particles, which have large particle diameters and which are used in Japanese Patent Laid-Open No. 2008-262171, have a wide particle size distribution and there are problems in properties to give the fluidity and the chargeability to a toner.
As described above, it is difficult to obtain silica particles which have large particle diameters and which can give stable chargeability and fluidity to a toner regardless of environment.